Streamflow data available on the web for various gaging stations are developed by regularly recording the height of the water surface (stage) at a station. This typically is accomplished through the use of a stage sensor, pressure transducer, manometer, or float in a stilling well with pipes leading to the stream. The sensor is connected to instrumentation which either continuously or at intervals records the stage. Streamflow (discharge) is calculated by applying the stage data to a stage-discharge curve or rating table developed for that station. Stage-discharge curves and rating tables are developed by physically measuring flow at various stages. Flow measurements are performed by determining the depth of water for numerous segments of a cross-section of the stream near the gaging station. The area of each of the segments is computed from the depth and the width of the segment. A current meter is then used to measure the velocity of flow at prescribed points in each of the segments. The velocities and areas are converted to discharge and related to the stage at the time of the measurement.

Floods and other high water events can have significant and permanent effects on the cross-section of a stream. In addition, debris in the stream, ice, and moss can temporarily affect the cross-section. Changes in the cross-section of a stream alter the relationship between stage and discharge. The development of accurate records requires that the stage-discharge curves be verified by periodically repeating the discharge measurements to define the effects of changes in the stream channel.

In addition, there are periods when stage is not recorded at a stream gaging station or when the data recorded are questionable and the resultant computed discharge is not representative of true discharge. If the recorder stops or otherwise fails to operate, data are not recorded. If the float freezes in the stilling well, the intakes to the well clog, the stream becomes iced over, or instrumentation malfunctions, the data recorded may not reflect the actual stage of the stream.

Provisional Data

Any initial computation or determination of discharge will result in some form of "provisional data," however, there are classes of "provisional" which represent differing qualities of values. Provisional data in any form are subject to change prior to publication. Because of the variety of factors which can introduce errors in provisional streamflow data, users are cautioned to consider carefully the nature of the information before using it for decisions that concern personal or public safety or the expenditure of money.

Preliminary Real Time Data The information on current streamflows (same day) which are available at this web site are preliminary. The information has been calculated based on raw data with little or no review to ensure the quality of the data. A computer regularly polls each of the stations to obtain information on stage and converts the information to discharge. This automated process does not allow confirmation of either the accuracy of the stage data used or whether the stage-discharge relationship has been altered. Therefore, there may be large differences between the real time data provided at this and other web sites and true discharge. Major adjustments in the data may be made prior to publication of the record. If the data are used for any purpose, a disclaimer should be included which describes the provisional nature of the data and the potential for significant errors.

Other Preliminary Data Data collected at each gaging stations are regularly reviewed by agency staff to verify the recorded stages and any changes in the stage-discharge relationship. Corrections or adjustments based on direct observations and discharge measurements are applied to compute the basic discharge record. Estimated values are developed for periods during which data are missing or determined to be not representative of true discharge. For example, if the recorded stage did not change during several days of cold weather and then dropped abruptly after warmer temperatures, staff would assume that ice had prevented movement of the float. The values for the period during which the float was frozen would be estimated based on known values at that station, runoff patterns at other nearby stations, and other available information which aids in making respresentative estimates.

If the initial evaluation of the data has been performed, these data will be representative, but not necessarily reflect final quantities. Users are cautioned that the review of the data to ensure accuracy may not have been completed. The data may be revised upon further analysis. A disclaimer indicating the provisional nature of the data should be included if the data are used for any purpose.

Final or Published Data

After the conclusion of the water year (October 1 to September 30), the streamflow data, if preliminary, are again reviewed or, if discharge has not yet been determined, the data development and review process is initiated. During this process, any previous computations of streamflows are evaluated and adjustments are made as necessary to reflect changes in the stage-discharge curve which may have occurred during the year. The streamflow data from stations in the drainage basin are compared to those from the other stations in the same or nearby basins and to weather records to determine if the flow patterns are consistent. Estimates of streamflows are verified or developed where the data are missing or appear to be erroneous. All data are considered to be provisional and are subject to change until the review process has been completed.

When the review process has been completed, the streamflow data are considered final and are published. These published records may be cited in research and used for other purposes. However, on rare occasions, errors are discovered after publication. When errors are discovered, the data are corrected in the electronic files available on the web and revisions are published as the first opportunity.

The accuracy of streamflow records depends primarily on (1) the stability of the stage-discharge relation and the frequency of discharge measurements; (2) the accuracy of measurements of stage, measurement of discharge, and (3) the interpretation of the records. The accuracy of a record is considered to be "excellent" if about 95 percent of the daily values are within 5 percent of true discharge, "good" if within 10 percent, "fair" if within 15 percent, and "poor" if not within 15 percent of true discharge.